In the perpendicular magnetic recording method, the easy axis of magnetization of the magnetic recording layer, which in conventional media is directed in the plane of the media, is directed in the direction perpendicular to the media; as a result, the demagnetizing field in the vicinity of the magnetization transition region, which is the boundary between recorded bits, is decreased, so that the higher the recording density, the more magnetostatically stable the recorded state, with improved resistance to thermal fluctuations; hence the method is appropriate for increasing areal densities.
When a soft magnetic underlayer, consisting of a soft magnetic material, is provided between the substrate and the perpendicular magnetic recording layer, the functions of so-called perpendicular double-layer media are obtained, and excellent recording performance can be obtained. At this time, the soft magnetic underlayer serves to provide a return path for the recording magnetic field from the magnetic head, and can improve the recording and reproduction efficiency.
In general, perpendicular magnetic recording media are configured with a soft magnetic underlayer provided on a substrate, on which are formed, in order, a non-magnetic underlayer which directs the easy axis of magnetization of the magnetic layer to be perpendicular to the plane of the substrate, a perpendicular magnetic recording layer consisting of a Co alloy, and a protective layer. However, in recent years, the WATE (for Wide Area Track Erasure, or Wide Adjacent Track Erasure) phenomenon has come to be recognized as a problem in perpendicular magnetic recording media. The WATE phenomenon is a problem specific to perpendicular magnetic recording media, and is a phenomenon in which, when a signal is recorded in a particular track, signals are demagnetized over a broad area extending for several μm from the recorded track. Methods for alleviating the problem, mainly through the structure or magnetic anisotropy of the soft magnetic underlayer, have been proposed (see for example Patent Reference Document 1).
It is also known that aligning the direction of easy axis of magnetization of the soft magnetic underlayer in the substrate radial direction is effective for solving the above problem. The following methods are proposed for realizing this magnetic structure;    1) depositing the soft magnetic underlayer under a magnetic field in the radial direction    2) stacking a soft magnetic layer and an antiferromagnetic film as a soft magnetic underlayer (see for example Patent Reference Document 2 and Patent Reference Document 3).
An example of a magnetic recording medium using a similar alloy constitution to this invention is proposed (see Patent Reference Document 4). However, this example is a longitudinal magnetic recording medium and the film thickness of a CoIr layer is thin.
The use of Ir in the soft magnetic underlayer is known but a perpendicular recording medium uses a porous material which is filled with a functional material (see Patent Reference Document 5). A perpendicular recording medium which uses Ir as a dividing layer between soft magnetic layers is known (see Patent Reference Document 6).
Patent Reference Document 1: Japanese Unexamined Patent Application, First Publication No. S58-166531
Patent Reference Document 2: Japanese Unexamined Patent Application, First Publication No. H06-103553
Patent Reference Document 3: US 2002/0028357
Patent Reference Document 4: Japanese Unexamined Patent Application, First Publication Number 2003-132515
Patent Reference Document 5: Japanese Unexamined Patent Application, First Publication Number 2004-237429
Patent Reference Document 6: Japanese Unexamined Patent Application, First Publication Number 2003-203326